Except for places where it is explicitly stated otherwise, in this application the term EFD will include those devices as introduced in U.S. application Ser. Nos. 657,867 and 730,110 and PCT Application Number PCT/U.S. Ser. No. 92/05883, and the term EFLCD will include the back light structures as introduced in U.S. application Ser. No. 812,730.
This invention relates in general to flat panel displays. This invention relates also to new cathode structures involving the FEAs (field emitter arrays) and the application of these new cathode structures in EFD (Electron Fluorescent Display), EFLCD (Electron Fluorescent Liquid Crystal Display) and MFD (Microtip Fluorescent Display) type direct matrix addressed type display.
EFD structures employ cathodes made of filaments whose operation is based on thermion emission. Thermion emission cathodes have become very popular since the days of vacuum tubes and this type of cathodes are still widely employed in technologies such as the cathode ray tube (CRT) and vacuum fluorescent device (VFD). There are many papers about this type of cathodes; their main strengths are the mature manufacturing technology and reliable operation. See P. S. Wagener, "The Oxide-Coated Cathode," 1951, Chapman & Hall Ltd. In the context of EFD applications, this filament cathode structure has the following weaknesses:
1. Due to the energy lost to the filament support by thermal conduction, the two ends and the center portion of each filament will not be at the same temperature. Since the rate of thermion emission is a sensitive function of the emitter body's temperature, the rate of emission at the two ends of the filament will be significantly lower than the center portion of the filament. This phenomenon is referred to as the cold terminal or cold end effect in U.S. application Ser. No. 730,110, and the above-referenced PCT application; it degrades the uniformity of the display and complicates the display control structure.
2. Due to large thermal inertia, the filament needs to be heated even when there is no need for it to emit electrons. For example, in a 1/100 duty factor EFD device, each portion of the filament needs to emit electrons only a few percent of the time, but the filament needs to maintain its high temperature constantly. This heating method causes a large amount of energy to be lost due to thermal radiation and conduction and degrades the luminous efficiency of the devices.
3. The operating temperature of the entire EFD device is raised due to the filament heating requirement. This elevated working temperature adversely affects the efficiency of the phosphor and the lifetime of various parts of the display system.
4. The filament needs to be spaced apart from other surfaces in order to maintain its temperature. This spacing increases the depth of the display device and complicates the structure and manufacturing process.
The cold cathode structure based on field emission principles, such as the microtip field emitter array (FEA) proposed by Spindt, is a solution to the above problems. See C. A. Spindt et al., "Physical Properties of Thin-Film Field-Emission Cathodes with Molybdenum Cones," pp. 5248, J. Appl. Physics, December 1976. These types of cathode structures have many nice features, such as high emission efficiency, high emission current, stable emission and simple control mechanism.
Companies such as Leti of France and SRI of the United States of America have demonstrated functional display devices based on direct addressed matrix of FEA cathodes of sizes up to a few inches in diagonal. See R. Meyer, "6-in. Diagonal Microtips Fluorescent Display For T.V. Application," pp. 374, IDRC 90 Proceedings, and C. A. Spindt et al., "Field-Emitter Arrays Applied to Vacuum Fluorescent Display," pp. 225, IEEE Trans. on Electron Devices, January 1989.
Coloray, a U.S. company, has also disclosed plans for making display devices based on this type of technology. See "Field Emission Display--Technology Review," Technical Note # 01, Coloray Display Corporation, 1990. As shown in FIG. 1A, these devices have a shared basic structure comprising a vacuum chamber between two face plates placed parallel to each other with a spacing of about 1 millimeter or less, wherein a transparent anode is placed on one of the face plates, a layer of phosphor dots placed on top of the anode, a matrix of field-emitting cathode dots between a set of column electrodes CE', and a set of row electrodes RE.
When a voltage is applied between a column electrode and a row electrode, the cathode dot located in the overlapping area between the two electrodes will emit electrons, so that the cathode dots are directly addressable through the two sets: CE', RE. Each cathode dot corresponds to a pixel of the display. Images are displayed by projecting and accelerating the electrons generated by the cathode dots toward the corresponding phosphor dots coated on top of the anode. The brightness of each phosphor dot is modulated by controlling the rate of electron emission by the corresponding cathode dots. There can be many other variations to the basic device structures. Some may involve focusing means between the cathode and the anode. However, no matter what variation of this structure, the image is formed by directly controlling the electron emission of each cathode dot. In other words, the electrons emitted by a matrix of cathode dots are directed towards corresponding parts of the display in directions normal to the face plates in response to the voltage applied between CE' and RE, essentially without deflection in directions parallel to the face plates; this manner of addressing is referred to herein as direct matrix addressing. We will refer to all these variations involving this operation principle as direct matrixed FEA displays.
Since the above-described FEA displays do not deflect the paths of the electrons for addressing, the microtip structure and its variations employed in these FEA displays require very high precision etching, patterning, and photolithography processes, production of functional FEA cathodes is expected to be very difficult. See Japan Patent Disclosure Number: JP 3-276542, December 1991. Due to such problems, the size of direct matrixed FEA displays is currently not expected to exceed a few inches in diagonal dimensions. For display devices based on direct matrixed FEA architecture, this problem of FEA cathode production quality and yield are likely to become the most important issues to be overcome.
Another area of the field emitter cathode technology that needs further improvements is the uniformity control of the electron emission rate. In the following references: R. Meyer, "6-in. Diagonal Microtips Fluorescent Display for T.V. Application," pp. 374, IDRC 90 Proceedings; T. Leroux et al., "Microtips Displays Addressing," pp. 437, SID 91 Digest; M. Borel et al., "Electron Source with Micropoint Emissive Cathodes and Display Means by Cathodoluminescence Excited by Field Emission Using Said Source," U.S. Pat. No. 4,940,916, June 1990; and as illustrated in FIG. 2A, it is proposed that a resistive layer is to be inserted between the electron emitter and the base electrode of the cathode. While the emission uniformity is improved, excessive electron emission is suppressed due to the loading of the inserted resistive layer as shown in FIG. 2B, so that this approach has some drawbacks as listed below:
1. The voltage drop over the inserted resistive layer pushes the control voltage Vgb, the gate to base potential difference, to be 80 V or higher. See R. Meyer, "6-in. Diagonal Microtips Fluorescent Display For T.V. Application," pp. 374, IDRC 90 Proceedings; T. Leroux et al., "Microtips Displays Addressing," pp. 437, SID 91 Digest; A. C. Lowe, "Microtip Field-Emission Display Performance Considerations," pp. 523, SID 92 Digest. This is undesirable because expensive high voltage drivers will be required to interface with the device.
2. The resistive layer only provides a moderate compensation to balance the current emission rate of the cathode. Higher level of compensation will require higher resistivity of the inserted layer which implies that even higher Vgb are required. This means either that the uniformity control of current emission will be too loose or the gate control voltage will be too high. Neither of these are desirable.
In reference Japan Patent Disclosure Number: JP 3-295138, December 1991 and FIG. 1B, a circuit made of two transistors and a capacitor is proposed to enhance the brightness of a direct matrixed FEA display. This circuit failed to address the two problems mentioned above. In addition, due to the fact that the control transistor is connected to the base of the emitter instead of the gate, the low current of each pixel and the generally sharp transition of FET transistor's I-V curve near its threshold voltage, the method proposed in the above-referenced Japanese disclosure can only be operated in switch mode, i.e., the circuit will either turn on or turn off the emission of the cathode. This drastically limits the capability of the display from achieving gray scales through analog modulation of the emission current and restricts the usefulness of this method in non-alphanumerical display applications.
None of the above-described systems is entirely satisfactory. It is thus desirable to provide an improved image display system in which the above discussed difficulties are avoided or reduced.